1. Field of the Invention
The present invention relates generally to sputter deposition of materials on substrate surfaces. More specifically, the present invention relates to methods and apparatus for measuring characteristics of a sputtering target and other surfaces within a sputtering vacuum chamber.
2. State of the Art
A thin film of metallic material may be deposited on a substrate using a sputter deposition process wherein a metallic target is attacked with ions, causing atoms or small particles of the target to be ejected from the target and deposited on the substrate surface. FIG. 1 illustrates a cross-sectional schematic of a conventional sputtering apparatus 10 comprising a vacuum chamber 12 having inner chamber walls 13, a gas inlet 14 and a gas outlet 16. The vacuum chamber 12 may further include a window 15 comprising a material that is transparent to predetermined wavelengths of electromagnetic radiation. The sputtering apparatus 10 further comprises a substrate support pedestal 24 and a metallic target 22 attached to a sputtering cathode assembly 18, each located within the vacuum chamber 12. The pedestal 24 may be configured to secure a substrate 26 thereto with a biasable electrostatic chuck, a vacuum chuck, a clamping structure, or a combination of methods. The substrate 26 may be transported to and from the pedestal 24 manually or with a robotic arm or blade (not shown).
During the sputtering process, the vacuum chamber 12 is filled with an inert gas, such as argon, through the gas inlet 14 and then reduced to a near vacuum through the gas outlet 16. The target 22 is negatively charged to cause electrons to be emitted from an exposed surface 23 of the target 22 and move toward an anode (not shown). A portion of the moving electrons strike atoms of the inert gas, causing the atoms to become positively ionized and move towards the negatively charged target 22. The electrons, inert gas atoms, and ions form a plasma which is typically intensified and confined over the target surface 23 by a magnetic field generated by a magnet assembly 20 located proximate the target 22. The magnet assembly 20 may comprise one or more permanent magnets or electromagnets located behind and/or to the side of the target 22. A portion of the ions discharging from the plasma strikes the target surface 23 at a high velocity, causing atoms or small particles of the target 22 material to be ejected from the target surface 23. The ejected atoms or small particles then travel through the vacuum chamber 12 until they strike a surface, such as the surface of the substrate 26, forming a thin metallic film thereon.
Residue deposits comprising the ejected atoms or small particles and byproducts are also deposited on the inner chamber walls 13 and other surfaces within the sealed vacuum chamber 12 during the deposition process. The accumulation of the residue deposits on the inner chamber walls 13 may be a source of contamination as a plurality of substrates 26 is successively processed in the vacuum chamber 12. Thus, the vacuum chamber 12 must be opened to atmosphere and cleaned after a predetermined amount of operation time has elapsed under vacuum or when contamination is detected on a substrate 26 that has undergone the deposition process. Opening and cleaning the vacuum chamber 12 is costly and time consuming. Therefore, it would be advantageous to clean the vacuum chamber 12 only when a predetermined amount of residue deposits have accumulated on the inner chamber walls 13 and other surfaces within the vacuum chamber 12.
The magnetic field formed over the target surface 23 by the magnet assembly 20 confines the electrons emitted from the target 22 to an area near the target surface 23. This greatly increases the electron density and the likelihood of collisions between the electrons and the atoms of the inert gas in the space near the target surface 23. Therefore, there is a higher rate of ion production in plasma regions near the target surface 23 where the magnetic field intensity is stronger. Varying rates of ion production in different plasma regions causes the target surface 23 to erode unevenly. Typically, the configuration of the magnet assembly 20 produces a radial variation of thick and thin areas, or grooves, within a diameter of the target surface 23. FIG. 2 illustrates a cross-sectional perspective view of a typical erosion profile of a cylindrical metallic target 22, such as the metallic target 22 shown in FIG. 1, which has been used in a sputtering process. FIG. 2 illustrates a target surface 23 before erosion has occurred as well as an eroded target surface 32 that has eroded unevenly across the length of a diameter of the target 22. Due to the geometry of a magnetic field surrounding the target 22, the target surface 32 has eroded nearly symmetrically about a center line 30 dividing the length of the diameter.
Referring now to FIGS. 1 and 2, the target 22 may comprise a rare metal, such as gold, platinum, palladium or silver, or may comprise, for example, aluminum, titanium, tungsten or any other target material conventionally employed in the semiconductor industry. Therefore, it is advantageous to consume as much of the target 22 material during sputter deposition processes as possible before replacing an eroded target 22. Further, replacing an eroded target 22 before the end of its useful life may be a difficult and time-consuming task. However, it is important to replace the target 22 before a groove “punches through” the target 22 material and exposes portions of the cathode assembly 18 to erosion, causing damage to the cathode assembly 18 and contaminating the sputtering apparatus 10. For example, the target 22 material in the area of grooves 28 shown in FIG. 2 may erode before the remainder of the target 22 material and expose the cathode assembly 18 to ionic bombardment from the surrounding plasma.
It may also be advantageous to replace or condition the sputtering target 22 when certain characteristics of the target surface 23 become degraded during the sputtering process. For example, the smoothness of the target surface 23 may degrade over time. The roughened target surface 23 may affect the consistency of the deposition formation on the substrate 26 and may also be an indication of the amount of target 22 consumption. Therefore, it may be advantageous to replace the target 22 when the target surface 23 reaches a predetermined roughness level.
As another example of degraded target surface 23 characteristics, certain targets 22, such as targets 22 comprising Ag2Se (hereinafter “silver selenide”), may exhibit hair-like growths or asperities (not shown) during the sputtering process. A portion of the asperities may be ejected from the target surface 23 during the plasma ion bombardment and land on substrate 26, forming defects therein. Typically, by the time the asperities have grown on the target surface 23 so as to create noticeable defects on the substrate 26, the target 22 is no longer useful and must be replaced. Therefore, to avoid forming defects on the substrate 26 and to prolong the useful life of the target 22, it may be advantageous to detect the asperities while the vacuum chamber 12 is under vacuum.
The useful life of a metallic sputtering target 22 is typically estimated by determining the cumulative deposition time for the target 22. A deposition time is chosen in an attempt to guarantee that the target 22 material will never be completely removed at any given location and may take into account the thickness of the target 22, the material used for the target 22, and the effect of intensifying and confining the plasma over the target surface 23 by a magnetic field generated by the magnet assembly 20 in a predetermined configuration. However, if the characteristics of the plasma distribution change due, for example, to reconfiguring the magnet assembly 20 to produce a magnetic field with a different geometry, the erosion of the target surface 23 may be changed and could result in localized enhanced metal removal and the possible punching through of target 22 to the cathode assembly 18 before the expiration of the estimated deposition time.
Directly measuring the characteristics of the target surface 23 or the vacuum chamber 12 is difficult and time consuming. Opening the vacuum chamber 12 to inspect the target surface 23 or inner chamber walls 13 requires several hours of idle time while the vacuum chamber 12 is baked out under post-vacuum inspection. Accurate measurement of the target surface 23 while the sputtering apparatus 10 is under vacuum is difficult because the gap distance d between the target 22 and the pedestal 24 may be as small as 25 millimeters. Typical measurement devices are too large to be inserted into the gap between the target 22 and the pedestal 24 to profile the target surface 23 while the vacuum chamber 12 is under vacuum. Further, measurement devices placed near the target 22 during a sputtering process may be damaged by exposure to metal deposition.
In view of the above-noted shortcomings in the art, it would be advantageous to prevent contamination from residue deposits on the inner chamber walls 13 and other surfaces and to prevent premature replacement, over-consumption or degradation of the target 22 by providing a technique and device to measure the inner chamber walls 13 and the target surface 23 while the vacuum chamber 12 is under vacuum.